Pulse shaping

Our model of deformable mirror is actuated on both side of the membranes designed to perform linear deformations suitable for ultrafast pulse compression and shaping. The use of the deformable mirror was demonstrated in the compression and shaping of an ultrabroadband NOPA with a spectrum spanning from 1µm – 1.7µm. The availability of femtosecond tailored pulses is a very important tool for the study of molecular processes in the time domain [1-3]. Novel schemes for the generation of fs pulse generation in spectral region less investigated such as UV and NIR have been proposed [4]. We recently demonstrated the compression down to less than two optical cycles OPA with an ultrabroadband spectrum (1.1µm-2.2µm) using an electrostatic membrane mirror actuated on one side of the membrane. Spectral regions so wide are difficult to be controlled using devices present in literature such as Liquid Crystals and Acusto Optic Modulators because of their extremely broad spectrum.

The device we propose is achromatic because it is a mirror and overcame the limits of weak deformation of electrostatic DMs introducing actuators on both sides of the membrane [5].

Fig. 1 illustrates a diagram of the mirror. The mirror is composed by a rectangular (45mm x 15mm) silver coated nitrocellulose membrane. The membrane is placed between two arrays of 30 linear electrodes.

The mirror is designed to be used in a 4f compressor as illustrated in Ref 5. The dispersion plane of the compressor is designed to fill the slit placed in bottom side electrodes (shown in light grey Fig 1). The slit for the beam has a size of 45mm x 2mm. Using this scheme both positive and negative deformation are possible. The maximum stroke applying 300V is +10µm using the Top side actuator and -9µm using the bottom side actuators.

Fig. 2, left columns shows the influence functions of the DMs of both sides of the mirror measured with a Twimann-Green interferometer. In order to generate the desired shapes pseudoinversion of the Influence functions matrix is used to calculate the set of voltages to be applied to the electrodes. Fig. 2 right column illustrates some example of interesting target shapes: triangular, three segments, third order and sinusoidal.

Fig. 2: left column: interferograms of the influence functions (Top block: top side electrodes. Bottom block: bottom side electrodes). Right column: measured interferograms of some interesting shapes realized with the doubly actuated linear deformable mirror (triangular, three segments, third order, sinusoidal).

The pulse shaping was demonstrated on a NOPA scheme with a spectrum from 1µm-1.7µm. In order to start our work of shaping we initially compress the pulse down to its transform limit value of about 8.5µm. The phase necessary for the compression was measured by a Second Harmonic FROG and then computed by pseudoinversion of the Influence Functions matrix.

Then the phase for the pulse shaping was carried out using the Gerchberg-Saxton iterative strategy [6] and fitted with the mirror influence functions. Fig. 3 shows the measured FROG trace and a train of two pulses 70fs spaced with a 20fs pulse length.

Fig. 3: measured FROG trace and double pulse with a time separation of 70fs with a pulse length of 20fs generated with the doubly actuated deformable mirror.